Though a neuron must be able to respond reliably to stimulation, it is equally important that it alters its response as a function of specific experiences. The molecular and cellular basis for this plasticity and how stimulus-specific changes in plasticity occurs is important to understand as it underlies both normal processes such as learning and memory as well as the disease states of addiction and depression. Our goal is to use the olfactory response of the genetically tractable nematode C. elegans to determine whether a small RNA regulatory pathway directs stimulus-specific neuronal plasticity. One means by which repeated stimulation alters neuronal responsiveness is via the changes in transcription elicited by epigenetic marks such as DNA methylation and histone modification (reviewed in1). In S. pombe, plants and Drosophila, chromatin marks have been shown to be directed by small RNAs2. These epigenetic changes are thought to regulate important developmental processes. Whether small RNAs can dynamically regulate epigenetic changes in neurons as a consequence of specific behaviors has not been examined. An attractive but completely untested hypothesis is that small RNAs might provide the guidance and specificity for epigenetic events that direct long-lasting changes in neuronal activity. As a first step towards testing this hypothesis, we asked whether genes required for RNA-interference (RNAi) might be required for neuronal plasticity in the anatomically simple but genetically powerful model organism, C. elegans. C. elegans is inherently attracted to specific odors which it senses using G-protein coupled receptors (GPCRs), however, its attraction is dampened if the odors are not accompanied by food. We term this experience-dependent dampening of the response to odor olfactory adaptation. The key switch that turns on olfactory adaptation is the entry of the cGMP-dependent protein kinase (PKG), EGL-4, into the nucleus of the odor-stimulated sensory olfactory neuron AWC (Lee et al., submitted). In our preliminary studies, we found that a specific class of small RNAs work with a gene encoding a chromatin associated protein, HPL-2, (a histone H3 lysine 9 tri-methyl binding protein), within the sensory neuron at the time of odor-exposure to promote adaptation. Both factors act downstream of EGL-4 nuclear entry and both factors act in the same genetic pathway for adaptation. Thus, our studies have raised the novel and exciting possibility that environmental stimuli can act via small RNAs to direct changes in chromatin. We propose to test this hypothesis by determining whether small RNAs and chromatin are central players in the adaptation process, how they function in adaptation and whether they can regulate transcription of candidate targets in response to prolonged odor-exposure. The significance of this work is that large scale epigenetic changes of the sort we are studying are found in models for addiction and depression (3,4). In the models of depression, these changes occur in the context of prolonged GPCR stimulation. How these changes occur, is unknown. Understanding the molecular details of the pathways by which neuronal stimulation is translated into chromatin marks is key to understanding these diseases.